The dominant intraoral x-ray radiographic imaging system detector for dentistry uses analog x-ray film optically-coupled to an x-ray scintillator screen (a film-screen unit) which is encased in a flexible, sealed (light-tight, impenetrable-to-fluids) soft packet film holder or carrier. The carrier may be made of a material such as molded Styrofoam or a similar light weight material that is radiographically-transparent. Dental x-ray film-screen units and carriers can be designed to fit a range of patient mouth sizes. The size of the film typically exceeds the size of the x-ray scintillator screen for handling purposes. The x-ray film, once processed, offers excellent spatial resolution with acceptable contrast resolution. The x-ray films can be displayed with an inexpensive light box and copied for distribution. A typical image acquisition scenario is to position the carrier in the patient's mouth, align the x-ray tube, and expose the x-ray film and x-ray phosphor screen held within the carrier to x-ray radiation. After exposure to x-rays the carrier is taken to a photographic dark room wherein the film is separated from the carrier by mechanical means and then developed in a film processor. Typically a vendor (for example KODAK) will provide the x-ray film, the film holder or carrier, the film processor and necessary chemicals and a light box for viewing developed films. Processing of films can take several minutes which is rarely an issue since the patient can continue to sit in a chair and read, etc. or other procedures can be initiated before the x-rays are reviewed. Developed films can be stored as an analog record or digitized and stored as a digital record. Analog film storage and retrieval/transportation expenses may become issues over time. Failure rates due to defective x-ray film are very low if film expiration dates are observed. Well-known limitations of this imaging format include the use of the x-ray scintillator screen which blurs the incident x-ray signal, the limited dynamic range of the film, and possible non-uniformities in the chemical film-development process.
An alternative detector technology to x-ray film is the intraoral digital x-ray camera (which still employees a scintillator screen) that is reusable. It offers acceptable (moderate) spatial resolution and very good contrast resolution (wide dynamic range). The digital readout is available relatively quickly and can be viewed on a monitor or an analog copy can be printed on film. Digital storage and retrieval/transportation are straightforward and cost-effective. Limitations of digital x-ray cameras include high cost per unit (limiting the number of units a typical dental office can own), the size and rigidity of the camera (patient discomfort), the ability to damage the camera physically or by radiation, a cable (communications, power) that sticks out of the patient's mouth (discomfort) and general hygiene issues. Recently a vendor has made three sizes of digital x-ray cameras available for intraoral radiography, the RVG 6100 System (KODAK), in an attempt to address the limitation imposed by using a single camera size for all applications. X-ray camera sizes are available for pediatric (22.2×30.8 mm2), general purpose (27.5×37.7 mm2), and bitewing radiographs (32.2×44.1 mm2) with resolution limits between 14-20 lp/mm. Note that the active (x-ray sensitive) detector areas are reduced to 17×22 mm222×30 mm2, and 27×36 mm2, respectively, due to packaging requirements that include a protective mount that also provides structural strength, a cable connection, and electronics. The cable is attached to either a USB connector or a battery/Wi-Fi unit. Digital cameras often incur patient tolerance issues since digital camera thicknesses range from 3-7 mm, The cable attachment introduces additional thickness to the camera. Wireless versions of digital cameras incorporate a battery and a transmitter, increasing the camera bulk. Intraoral digital x-ray cameras are often advertised as a means of lowering patient radiation dose relative to x-ray film-based imaging.
Storage phosphor detectors that utilize granular-particle storage phosphors have been used for general diagnostic x-ray radiography since the 1980s, typically in the form of large area rigid plates for applications ranging from chest x-rays to x-ray mammography (Rowlands J., Phys. Med. Biol. 47, pp. 123-166, 2002).A well-known limitation for current implementations of storage phosphor plates used in x-ray mammography is that they offer relatively poor spatial resolution compared to mammography x-ray film-screen units. Digital dental imaging spatial resolution requirements are comparable or exceed those of digital mammography. Improving spatial resolution typically requires thinner storage phosphor plates (actually screens, as used in digital dentistry), resulting in reduced x-ray detection efficiency and increased patient x-ray radiation dose. Market demand for digital dentistry systems based on conventional storage phosphor screen technology has been limited compared to CCD and CMOS-based digital camera systems. This poor market acceptance is despite the obvious advantages of offering much thinner detector (about 1 mm thick) with limited flexibility and a greater range of intraoral sizes than digital cameras, ranging from 22×31 mm2 to 48 ×54mm2. More recently, the technologies used to develop ceramic and nanoparticle (and nano-composite) ceramic scintillators for applications such as nuclear medicine and PET scintillators have been applied to produce large area, relatively transparent nanoparticle storage phosphor ceramic plates (also referred to as storage phosphor plates) capable of extremely high spatial resolution (much higher than 20 1p/mm) and excellent contrast resolution for applications such as x-ray mammography (Edgar A., et al., Current Applied Physics 6, pp. 399-402, 2006; Chen G., et al., Journal of Non-crystalline Solids 352, pp.610-614, 2006; Johnson J., Schweizer S, J. Am. Ceram. Soc. 90[3] pp. 693-698, 2007). A typical readout mechanism involves scanning the storage phosphor plate with a fine spot optical beam from a source such as a continuous or pulsed optical laser or LED and coupling the fluorescent light (via fiber optics or other conventional optical coupling means) from the discharged storage phosphor plate to an amplified photodetector such as a photomultiplier tube (PMT). The PMT has little or no response to the scanning beam wavelength due to added filtration and photocathode insensitivity at the long wavelengths used for the scanning beam.
A desirable dental x-ray detector would incorporate (or improve on) the favorable properties of both x-ray film-screen and x-ray camera detectors (cost-effective, reliable, reusable, excellent spatial and contrast resolution, large dynamic range, digital readout). In addition, the dental x-ray detector should offer comparable or superior x-ray detection efficiency to film-based detectors or x-ray cameras and thus reduce patient risk. Preferably the active detector area should be nearly 100% of the total detector area so that the carrier can be of a comparable size to the active x-ray detector. The dental x-ray detector would be sufficiently robust such that failure rates due to damage or exposure limits are small or negligible.